1. Field of the Invention.
The invention in general relates to security systems and in particular a wireless security system having one more detector/sending units for reporting the existence of a condition to a central receiving unit.
2. Description of the Prior Art
Security systems which include one or more sending units which transmit coded radio frequency (r-f) signals to a central receiving unit which decodes the signals to produce an alarm or other indication of a condition at the sending unit location are well known. The condition may be the existence of a fire, an intrusion, an emergency, the presence of water or other fluid, or other condition desired to be monitored. Or the condition may be the status of the sending unit, such as the condition of its battery or other sensor status. The term "security system" as used herein is intended to include any such system that sounds an alarm or reports on one or more of the above conditions. Generally, the information sent will also include the identity or location of the sending unit. A major problem with r-f or wireless security systems is the lack of reliability of the communicated data. The information or the condition, status, location etc. is generally transmitted serially as a string of digital data bits modulated on the r-f carrier wave which is received and demodulated by the central receiving unit to provide a digital data string to a processing circuit which analyzes the data. Because of the nature of r-f communication, noise can disturb this process by causing unwanted transitions in otherwise valid transmitted data or by generating apparent data that is actually only noise. Since the processing circuitry analyzes the received data for information about the status of the various sensors, noise in the data can cause a system to either reject a valid transmission or to falsely report an alarm status for one of the sensors. Previous attempts to solve this problem have involved transmitting the data several times and requiring the processing circuitry to receive multiple, identical data strings before reporting an alarm condition. This results in inefficient use of transmission time, leading to problems with battery life, clash (or collision) of transmissions from different sending units and meeting FCC regulations on net broadcast energy. This invention discloses a new approach for solving the noise problem in security systems involving filtering the received signals to remove the noise and analysis of the signals to reject signals which are too noisy to be filtered.